Propeller impact detection and force reduction

ABSTRACT

A commanded control signal is compared against an adaptive control signal in order to detect a rotor strike by a rotor included in an aircraft, wherein the adaptive control signal is associated with controlling the rotor and the adaptive control signal varies based at least in part on the commanded control signal and state information associated with the rotor. In response to detecting the rotor strike, a control signal to the rotor is adjusted in order to reduce a striking force associated with the rotor.

CROSS REFERENCE TO OTHER APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 15/877,047, entitled PROPELLER IMPACT DETECTION AND FORCEREDUCTION filed Jan. 22, 2018, which claims priority to U.S. ProvisionalPatent Application No. 62/595,963, entitled PROPELLER IMPACT DETECTIONAND FORCE REDUCTION filed Dec. 7, 2017, each of which is incorporatedherein by reference for all purposes.

BACKGROUND OF THE INVENTION

New types of lightweight and ultra-lightweight aircraft are beingdeveloped for recreational use, use by novice pilots, and/or for use innew flying environments (e.g., they can take off and land from abackyard). In some of these aircraft, the rotors have no shield or bladeand are therefore exposed. New techniques to detect a rotor strike andreduce the rotor's force in response to detection of a rotor strikewould be desirable. Although strike detection techniques may exist forother applications, it would be desirable if techniques werelightweight, low cost, and/or better suited to the various needs and/ordesign considerations of an aircraft.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the invention are disclosed in the followingdetailed description and the accompanying drawings.

FIG. 1 is a flowchart illustrating an embodiment of a process to detecta rotor strike and perform an adjustment to reduce a striking force inresponse to a detected rotor strike.

FIG. 2A is a diagram illustrating a top view of a multicopter embodimentwith unshielded rotors.

FIG. 2B is a diagram illustrating a front view of a multicopterembodiment with unshielded rotors.

FIG. 3A is a block diagram illustrating an embodiment of rotor strikedetection system which uses three phase-shifted sinusoidal signals asthe adaptive control signal.

FIG. 3B is a graph illustrating an embodiment of an increase in anadaptive control signal (comprising phase-shifted sinusoidal signals)due to a commanded control signal.

FIG. 3C is a graph illustrating an embodiment of an increase in anadaptive control signal (comprising phase-shifted sinusoidal signals)due to a rotor strike.

FIG. 4 is a diagram illustrating an embodiment of an adaptivemeasurement threshold which is based on a commanded control signal.

FIG. 5 is a diagram illustrating an embodiment of an adaptive sinusoidalreference signal which is based on a commanded control signal.

FIG. 6 is a flowchart illustrating an embodiment of a process to comparea commanded control signal against an adaptive control signal in orderto detect a rotor strike.

FIG. 7A is a diagram illustrating an embodiment of electrical connectorsand pull-down resistors which are used to adjust a control signal to therotor in order to reduce a striking force.

FIG. 7B is a diagram illustrating an embodiment of built-in switches ina switched converter which are used to adjust a control signal to therotor in order to reduce a striking force.

FIG. 8A is a flowchart illustrating an embodiment of a process to adjusta control signal to the rotor in order to reduce a striking force usinga pull resistor.

FIG. 8B is a flowchart illustrating an embodiment of a process to adjusta control signal to the rotor in order to reduce a striking force usingbuilt-in switches in a switched converter.

FIG. 9A is a diagram illustrating an embodiment of a single altitudethreshold which is used in responding to a detected rotor strike.

FIG. 9B is a diagram illustrating an embodiment of two altitudethresholds which are used in responding to a detected rotor strike.

FIG. 10A is a flowchart illustrating an embodiment of a process toadjust the control signal to the rotor in order to reduce the strikingforce associated with the rotor using altitude and an altitudethreshold.

FIG. 10B is a flowchart illustrating an embodiment of a process toadjust the control signal to the rotor in order to reduce the strikingforce associated with the rotor using altitude and an altitude thresholdwhere a pilot is permitted to restart the rotor.

FIG. 10C is a flowchart illustrating an embodiment of a process toadjust the control signal to the rotor in order to reduce the strikingforce associated with the rotor using two altitude thresholds when thereis a parachute system.

DETAILED DESCRIPTION

The invention can be implemented in numerous ways, including as aprocess; an apparatus; a system; a composition of matter; a computerprogram product embodied on a computer readable storage medium; and/or aprocessor, such as a processor configured to execute instructions storedon and/or provided by a memory coupled to the processor. In thisspecification, these implementations, or any other form that theinvention may take, may be referred to as techniques. In general, theorder of the steps of disclosed processes may be altered within thescope of the invention. Unless stated otherwise, a component such as aprocessor or a memory described as being configured to perform a taskmay be implemented as a general component that is temporarily configuredto perform the task at a given time or a specific component that ismanufactured to perform the task. As used herein, the term ‘processor’refers to one or more devices, circuits, and/or processing coresconfigured to process data, such as computer program instructions.

A detailed description of one or more embodiments of the invention isprovided below along with accompanying figures that illustrate theprinciples of the invention. The invention is described in connectionwith such embodiments, but the invention is not limited to anyembodiment. The scope of the invention is limited only by the claims andthe invention encompasses numerous alternatives, modifications andequivalents. Numerous specific details are set forth in the followingdescription in order to provide a thorough understanding of theinvention. These details are provided for the purpose of example and theinvention may be practiced according to the claims without some or allof these specific details. For the purpose of clarity, technicalmaterial that is known in the technical fields related to the inventionhas not been described in detail so that the invention is notunnecessarily obscured.

Various embodiments of a technique to detect a rotor strike and respondaccordingly are described herein. In some embodiments, the process isperformed by an aircraft, such as a multicopter. In some embodiments,the technique includes comparing a commanded control signal against anadaptive control signal in order to detect a rotor strike by a rotorincluded in an aircraft, where the adaptive control signal is associatedwith controlling the rotor and the adaptive control signal varies basedat least in part on the commanded control signal and state informationassociated with the rotor; in response to detecting the rotor strike, acontrol signal to the rotor is adjusted in order to reduce a strikingforce associated with the rotor.

FIG. 1 is a flowchart illustrating an embodiment of a process to detecta rotor strike and perform an adjustment to reduce a striking force inresponse to a detected rotor strike. In some embodiments, the process isperformed by an aircraft (e.g., a multicopter) where the rotors have noshield or guard to prevent the rotors from striking anything. In someapplications, such aircraft are not taking off and landing at airportsand therefore space around the aircraft may be less controlled comparedto airports during takeoff and landing. In one example, the aircraft isa single-seat, recreational multicopter and the owner takes off andlands from his/her backyard (e.g., where the multicopter may strike aperson, a pet, a tree, etc.).

At 100, a commanded control signal is compared against an adaptivecontrol signal in order to detect a rotor strike by a rotor included inan aircraft, wherein the adaptive control signal is associated withcontrolling the rotor and the adaptive control signal varies based atleast in part on the commanded control signal and state informationassociated with the rotor. In one example where the aircraft is a mannedaircraft, the commanded control signal is based on a pilot'sinstructions or commands (e.g., which are received via one or more handcontrols, such as a joystick, a thumbwheel, etc.) and/or a flightcontroller (e.g., the pilot is not touching the joystick and the flightcomputer performs small adjustments to keep the multicopter hover in airin the same position). In some embodiments, the aircraft has anautomated flight controller (e.g., in an unmanned aircraft withautonomous flight capabilities) and the commanded control signal comesfrom the automated flight controller. In some examples described below,the commanded control signal conveys or otherwise indicates a desiredrotations per minute (RPMs) or a desired torque (e.g., associated with aparticular rotor in the case of a multicopter).

Generally speaking, a rotor strike is detected or declared at step 100when the adaptive control signal and the commanded control signal do notindicate the same or similar thing (e.g., similar RPMs or similartorques). For example, suppose that the pilot is not holding thejoystick and so the flight controller only makes small changes to thecommanded control signal so that the aircraft hovers in air at constantposition. If, during this period, a very large spike or increase (e.g.,in torque) is observed in the adaptive control signal, this would beflagged as a rotor strike (at least in this example) because there wasno corresponding indicator in the commanded control signal that anincrease (e.g., in torque) was requested by the pilot and/or the flightcontroller. Presumably, the rotor has hit something and this resistancewas observed by the control system and/or feedback loop, causing theadaptive control signal to increase.

In contrast, if the pilot (who previously had not been holding thejoystick) grabbed the joystick in order start moving (e.g., quickly),this increase or change would be reflected in the adaptive controlsignal, and a spike or increase by the adaptive control signal would notbe flagged or otherwise identified as a rotor strike because there wouldbe a corresponding increase or indication in the commanded controlsignal. More detailed examples of how rotor strike detection isperformed are described below.

At 102, in response to detecting the rotor strike, a control signal tothe rotor is adjusted in order to reduce a striking force associatedwith the rotor. In some embodiments, the control signal which isadjusted is the adaptive control signal. Alternatively, in some otherembodiments, the commanded control signal may be the control signalwhich is adjusted at step 102.

In some embodiments, the adjustment to the control signal at step 102actively stops the rotation of the rotor (e.g., by applying a brakinginstruction or braking force). For example, the adjustment to thecontrol signal may cause the control signal to indicate that the rotorshould rotate in the opposite direction (e.g., acting like a brakingforce on the rotor). Alternatively, the control signal to the rotor canbe changed to a neutral value, where the rotor is instructed to rotatein neither direction. In that scenario, inertia will cause the rotor tocontinue rotating in its current direction but the striking force willbe reduced because the control signal is no longer instructing the rotorto continue rotating.

The following figures describe an exemplary aircraft which may use theprocess of FIG. 1 to detect rotor strikes and reduce a striking forcegenerated by a corresponding rotor accordingly.

FIG. 2A is a diagram illustrating a top view of a multicopter embodimentwith unshielded rotors. In the example shown, multicopter 200 is asingle seat aircraft with 10 rotors where the rotors have no guard orshield (e.g., for cost and/or weight reasons) to prevent the rotors fromcoming into contact with something, either while the multicopter is onthe ground or in the air. In multicopter 200, the rotors are mounted tothe multicopter at fixed positions or angles. To maneuver, the variousrotors are rotated independently of one another at different speeds.Although not necessarily shown from this top view, the rotors are allslightly angled (i.e., they do not rotate in a level horizontal plane)to make the multicopter more maneuverable.

In this example, multicopter 200 takes off and lands vertically. Thisfeature eliminates the need for a runway (e.g., the multicopter can takeoff and land in a field, in a backyard, etc.). After ascendingvertically during takeoff, the aircraft can switch to a forward flightmode of flight (e.g., once a desired cruising altitude is reached) wherethe aircraft flies at a constant or steady altitude. If desired, themulticopter can hover at a constant or steady position midair.

For redundancy and to avoid a single point of failure (which isdesirable in aircraft design), each of the 10 rotors in this example hasits own set of hardware and/or electrical components which monitor thecorresponding rotor for a rotor strike. In this example, each set ofelectronics independently performs the process of FIG. 1 on itsrespective rotor. That way, even if one set of electronics (e.g.,associated with one rotor) fails, a rotor strike can still be detectedfor another rotor and the striking force for that rotor can be reduced.

FIG. 2B is a diagram illustrating a front view of a multicopterembodiment with unshielded rotors. FIG. 2B shows the same exemplarymulticopter 200 from FIG. 2A but from a different view. From this frontview, the height of the rotors relative to the ground is more readilyapparent and in this example the rotors are on the order of 2-4 feet offthe ground. At this rotor height (e.g., which is less than the averageheight of an adult), a rotor strike is possible when the multicopter ison the ground and the rotors are rotating. In some cases, a rotor strikeoccurs when the multicopter is airborne but is relatively low to theground. A rotor strike can also occur at higher altitudes. For example,the aircraft could be 20 feet off the ground or higher and one or moreof the rotors could strike a tree, a power or telephone line, a bird,etc.

In the exemplary multicopter shown here, the adaptive control signal(not shown) includes a plurality of phase-shifted sinusoidal signals.The following figures describe an example of this.

FIG. 3A is a block diagram illustrating an embodiment of rotor strikedetection system which uses three phase-shifted sinusoidal signals asthe adaptive control signal. In this example, the adaptive controlsignal (e.g., used at step 100 in FIG. 1 to detect a rotor strike)comprises three phase-shifted sinusoidal signals (300) which arereferred to in the following figures as the A signal, the B signal, andthe C signal. The signals are phase shifted with respect to each otherwhere the B signal has a 120° phase offset from the A signal and the Csignal has a 240° phase offset from the A signal.

The phase-shifted sinusoidal signals (300) are generated in this exampleby a motor controller (302) which includes a rotor strike detector (308)and drive and control block (309). The motor controller (and morespecifically, the drive and control block) inputs a commanded controlsignal (310), state information from sensors (304), and a strikeindication signal from rotor strike detector 308 in order to generatethe three phase-shifted sinusoidal signals (300). For example, if astrike is signaled, then the adaptive control signals may be set tovalues which will “back off” the rotor's striking force (e.g., either byactively “braking” the rotor, or by exerting no new or additional torqueso that the rotor eventually comes to a stop).

The phase-shifted sinusoidal signals (300) are passed to rotor 306 andcontrol the rotation of the rotor (306). For example, as the magnitudeand/or frequency of the phase-shifted sinusoidal signals increases, therotor (306) attempts to rotate faster and/or with more torque. In someembodiments, the phase-shifted sinusoidal signals (or, more generally,the adaptive control signal) can indicate directionality of rotation.For example, one direction of rotation may be associated with normaloperation (e.g., where in this direction the rotors provide thenecessary lift for the aircraft to fly) and the other direction ofrotation may be used as a brake (e.g., in the event a rotor strike isdetected and the system is trying to bring the rotor to a stop as fastas possible).

The rotor (306) causes changes to the state of the rotor and thesechanges are measured by sensors (304) which output state information. Invarious embodiments, the state information from the sensors may relateto a measured RPM of the rotor, a measured torque of the rotor, etc. Invarious embodiments, the sensors may include GPS, gyroscopes,accelerometers, etc. For simplicity, only a single rotor is shown herebut as FIG. 2A and FIG. 2B show, an aircraft may include more than onerotor and rotor strike detector and/or rotor force mitigation componentsmay be duplicated for redundancy and/or to avoid a single point offailure.

To detect a rotor strike, rotor strike detector (308) inputs thecommanded control signal (310) and the phase-shifted sinusoidal signals(300). Rotor strike detector (308) is one example of a block orcomponent which performs step 100 in FIG. 1. In some embodiments, ifthere is some change in the phase-shifted sinusoidal signals (e.g.,corresponding to an increase in the amount of torque and/or reduction inrotations per minute) which is not correspondingly reflected in thecommanded control signal, then a rotor strike is flagged or otherwiseidentified.

In some embodiments, rotor strike detector (308) may be implemented as afield-programmable gate array (FPGA), which would tend to permit morecomplex and/or powerful rotor strike detection processing operations.Alternatively, the rotor strike detector may be implemented using amicroprocessor (e.g., where the microprocessor is shared between a rotorstrike detection process and other flight-related processes where lesscomplex and/or less powerful rotor strike detection processingoperations are supported).

Depending upon the state information and the commanded control signal,the motor controller (302) will adjust the phase-shifted sinusoidalsignals as or if needed. The following figures show some examples ofthis.

FIG. 3B is a graph illustrating an embodiment of an increase in anadaptive control signal (comprising phase-shifted sinusoidal signals)due to a commanded control signal. In this example, the A signal (322),B signal (324), and C signal (326) are examples of adaptive controlsignals 300 in FIG. 3A. In this example, the magnitudes of signals 322,324, and 326 increase, as do the frequencies of those signals, inresponse to the increase in the level or value of the commanded controlsignal (320). For example, a pilot may instruct the rotor or aircraft togo faster, and the rotor or aircraft responds accordingly.

Before time t0, the level of the commanded control signal (320) is at afirst level (L0). In response to that signal level (and because thesystem is behaving perfectly in this example), the magnitudes of thethree phase-shifted sinusoidal signals (322, 324, and 326) are at thesame magnitude (i.e., L0). (For convenience and simplicity, all of thesignals are shown at the same level, but the phase-shifted sinusoidalsignals may have different magnitudes or signal levels compared to thecommanded control signal.)

At time t0, the commanded control signal (320) increases to a second,higher level (L1). In response to that increase, the magnitudes andfrequencies of the A signal (322), B signal (324), and C signal (326)increase so that they match the level of the commanded control signal(320) at L1. It is clear that in this example, the observed change inthe magnitude and frequency of the A signal (322), B signal (324), and Csignal (326) is due to the commanded control signal and not (as anexample) to counter an external force.

Alternatively, the magnitude or level of the phase-shifted sinusoidalsignals could change because of a rotor strike. The following figureshows an example of this.

FIG. 3C is a graph illustrating an embodiment of an increase in anadaptive control signal (comprising phase-shifted sinusoidal signals)due to a rotor strike. In this example, the commanded control signal(340) remains at a same level (L0) the entire time. For convenience, thesame time reference (t0) is shown in this figure as in the previousfigure. In this example, the rotor encounters some resistance due to arotor strike. In response, even though the commanded control signal(340) remains at the same level, the magnitudes and frequencies of the Asignal (342), B signal (344), and C signal (346) increase so that therotor will attempt to apply more torque in order to counter theresistance and satisfy the commanded control signal. It is noted thatthe response to a rotor strike is unpredictable and implementationdependent and this is just one example of how a rotor strike wouldaffect an adaptive control signal comprising phase-shifted sinusoidalsignals.

Returning to step 100 in FIG. 1, a scenario like the one shown in FIG.3B shows an example where a rotor strike would not be declared (e.g.,because the change in the phase-shifted sinusoidal signals correspondsand/or is corroborated by the change in the commanded control signal).In contrast, FIG. 3C shows an example of something that would be flaggedas a rotor strike (e.g., because the change in the frequency and/oramplitude of the phase-shifted sinusoidal signals is not due to or inresponse to some change in the commanded control signal).

To differentiate between the two scenarios shown in FIGS. 3B and 3C, ameasurement threshold may be used to obtain some metric or measurementwhich in turn is used to decide whether to declare a rotor strike. Thefollowing figure shows an example of a measurement threshold which isbased on the commanded control signal.

FIG. 4 is a diagram illustrating an embodiment of an adaptivemeasurement threshold which is based on a commanded control signal. Inthe example shown, the value of the measurement threshold is based uponthe value of the commanded control signal. For this reason, theexemplary measurement threshold shown here is an adaptive measurementthreshold as opposed to a fixed measurement threshold. In someembodiments, a fixed measurement threshold is used (e.g., where themeasurement threshold does not vary or change with the commanded controlsignal).

In this example, to measure the degree to which a (phase-shifted)sinusoidal signal (e.g., signal 322, 324, or 326 in FIG. 3B or signal342, 344, or 346 in FIG. 3C) is responding to resistance from the rotor(e.g., due to a rotor strike) as opposed to a change in the commandedcontrol signal, the measurement threshold takes into account the valueof the commanded control signal. As shown in FIGS. 3B and 3C, anincrease in a (phase-shifted) sinusoidal signal could come from eitherresistance or the commanded control signal.

When the commanded control signal corresponds to a low RPM value (or,alternatively, a low torque value), the measurement threshold is set tofirst, lower value (400). When the commanded control signal correspondsto a high RPM (or, alternatively, torque) value, the measurementthreshold is set to second, higher value (410). Two scenarios are shownhere but naturally more than two measurement threshold levels or valuesmay be used.

The measurement threshold is used to measure the degree or amount thatone or more (phase-shifted) sinusoidal signals (e.g., which make up theadaptive control signal) do not correspond to the current commandedcontrol signal. To preserve the readability of the graph, only the Asignal is shown here; the corresponding B signal and C signal are notshown. Signal 402 shows an A signal when the commanded control signalcorresponds to a low RPM value. An estimate or measurement of the areabounded by measurement threshold 400 and A signal 402 is performed,which corresponds to shaded region 404. This area (404) is comparedagainst a second threshold (referred to herein as a detection threshold,not shown) and if the area exceeds this second/detection threshold, arotor strike is declared.

The same decision making and/or comparison is performed when thecommanded control signal corresponds to a high RPM value. In thisscenario, dotted region 414 is a measurement of the area bounded by themeasurement threshold (410) when the commanded control signalcorresponds to a high RPM value and the A signal (412) when thecommanded control signal corresponds to a high RPM value. This area(414) is then compared against a detection threshold (not shown) and ifthe detection threshold is exceeded, a rotor strike is declared. To putit another way, by adapting the measurement threshold to the commandedcontrol signal, the strike detection threshold takes into accountinstances when the commanded control signal might be causing theadaptive control signal to change to a higher RPM or higher torquevalue.

In some embodiments, the detection threshold is set to a value whichdifferentiates between normal and/or acceptable amounts of resistancewhich are relatively small versus large amounts of resistance which areprobably indicative of a rotor strike. For example, it would bedesirable to differentiate between changes in the adaptive controlsignal due to noise versus changes in the adaptive control signal due toa rotor strike (e.g., where the rotor at least temporarily slows downand/or is impeded).

Returning briefly to FIG. 3A, in some embodiments, rotor strike detector308 monitors the commanded control signal (310) in real-time and updatesan internal measurement threshold in real-time. This measurementthreshold is then used to detect a rotor strike as described above. Insome embodiments, rotor strike detector 308 uses a lookup table to mapthe value of the commanded control signal to a corresponding measurementthreshold.

In this example, all of the signals are monitored, but a rotor strike isable to be declared even if only one of the signals has been processedand/or exhibits strike-like characteristics or properties. Forapplications where a fast decision is desired, this may be attractive.Alternatively, if accuracy is desired, more signals which exhibitstrike-like characteristics (e.g., at least two signals have to exhibitstrike-like characteristics) and/or exhibiting these characteristicsover a longer period may be required before declaring a rotor strike.Although more accurate, this may be slower and depending upon theapplication or design constraints the appropriate technique may be used.

In some embodiments, instead of using a measurement threshold to obtaina metric or value (e.g., which is then compared against a detectionthreshold), a single threshold is used and threshold crossing is used todeclare a rotor strike. For example, a rotor strike would be declared ifsignal 402 (412) crossed threshold 400 (410). In various embodiments,threshold crossing detecting may be used in combination with a fixedthreshold or an adaptive threshold (e.g., some Δ above the commandedcontrol signal).

This example of using a measurement threshold is merely one example ofhow a rotor strike may be detected. In some embodiments, some deviationfrom a (e.g., sinusoidal) reference signal (e.g., which varies dependingupon the commanded control signal) may be used to detect a rotor strike.The following figure shows an example of this.

FIG. 5 is a diagram illustrating an embodiment of an adaptive sinusoidalreference signal which is based on a commanded control signal. In thisexample, the magnitude of the (sinusoidal) reference signal is selectedbased on the level or value of the commanded control signal. When thecommanded control signal corresponds to a low RPM (or, alternatively,torque) value, a sinusoidal reference signal with a lower magnitude(500) is selected. When the commanded control signal corresponds to ahigh RPM value, a sinusoidal reference signal with higher magnitude(510) is selected.

The A signals (502 and 512) are then compared against their respectivereference signals. Shaded area 504 shows the degree or amount that Asignal 502 (when the commanded control signal is at a low RPM value)deviates from reference signal 500 (also when the commanded controlsignal is at a low RPM value). Dotted area 514 shows the degree oramount that A signal 512 (when the commanded control signal is at a highRPM value) deviates from reference signal 510 (also when the commandedcontrol signal is at a high RPM value). As before, a measurement and/orestimate of the areas (504/514) is compared against a detectionthreshold (not shown) and a rotor strike is declared if the area exceedsthe detection threshold.

As in the previous example, the commanded control signal may bemonitored and the magnitude of the sinusoidal reference signal may beadjusted in response to any changes in the commanded control signaland/or a lookup table may be used to map the commanded control signal toa magnitude to use for the reference signal. In some embodiments, usinga (e.g., sinusoidal) reference signal is faster at detecting a rotorstrike than using a measurement threshold in combination with adetection threshold.

In some embodiments, a non-sinusoidal reference signal is used. Forexample, a sawtooth reference signal may be used where the signal ismade of various lines for different segments.

The following figure describes the above examples more generally and/orformally in a flowchart.

FIG. 6 is a flowchart illustrating an embodiment of a process to comparea commanded control signal against an adaptive control signal in orderto detect a rotor strike. In some embodiments, this process is used atstep 100 in FIG. 1. Rotor strike detector 308 shows one example of acomponent which may perform the process of FIG. 6.

At 600, a reference signal is determined based at least in part on thecommanded control signal. For example, measurement thresholds 400 and410 from FIG. 4 are examples of a reference signal where the value ofthe measurement threshold varies depending upon the RPM (or,alternatively, torque) value indicated by the commanded control signal.In FIG. 5, sinusoidal reference signals 500 and 510 (e.g., where themagnitude of the reference signal depends upon the RPM or torque valueindicated by the commanded control signal) show another example of areference signal which is determined at step 600.

At 602, a degree to which the adaptive control signal, which includes asinusoidal signal, deviates from the reference signal is determined. Forexample, in FIG. 4, an estimate or measurement of shaded area 404 ordotted area 414 is performed. In the example of FIG. 5, shaded area 504or dotted area 514 is measured or otherwise estimated.

At 604, the degree is compared against a detection threshold. Forexample, shaded area 404 or dotted area 414 in FIG. 4 is comparedagainst some detection threshold. Or, shaded area 504 or dotted area 514in FIG. 5 is compared against the detection threshold.

At 606, in response to the degree exceeding the detection threshold, itis declared that the rotor strike has been detected. As described above,the detection threshold may be used to differentiate between relativelylow and/or typical amounts of deviation (e.g., due to relatively lowand/or acceptable amounts of resistance, for example, due to noise orsmall errors) versus relatively high and/or atypical amounts ofdeviation (e.g., due to a rotor strike). Alternatively, if the detectionthreshold is not exceeded, then no rotor strike is declared.

Returning briefly to step 102 in FIG. 1, in response to detecting therotor strike, a control signal to the rotor is adjusted in order toreduce a striking force associated with the rotor. The following figuresshow some examples of this.

FIG. 7A is a diagram illustrating an embodiment of electrical connectorsand pull-down resistors which are used to adjust a control signal to therotor in order to reduce a striking force. In the example shown, motorcontroller 700 inputs a commanded control signal (not shown) and outputsan adaptive control signal in the form of three phase-shifted sinusoidalsignals (702).

The phase-shifted sinusoidal signals are passed to electrical connectors704. During normal operation, the electrical connectors pass what isobserved at the input (i.e., phase-shifted sinusoidal signals 702)through to the output (i.e., rotor inputs 706). In this scenario, therotor (708) would receive and be controlled by the phase-shiftedsinusoidal signals (702).

However, if a rotor strike is detected, the electrical connectors will(e.g., at least electrically) disconnect the phase shifted sinusoidalsignals (702) from the rotor inputs (706) so that those signals are nolonger driven. In the absence of any driving signal (e.g., when theelectrical connectors (704) have disconnected the phase-shiftedsinusoidal signals (702) from the rotor inputs (706)), the pull-downresistors (710) will pull the rotor inputs (706) to ground. Pull-upresistors are similar to pull-down resistors except they are connectedto power as opposed to ground and in some embodiments a pull-up resistoris used to stop a rotor in a manner similar to that shown here. Thevalue set by the pull-down resistors at the rotor input (at least inthis example) corresponds to either a neutral value (e.g., where nobraking force is applied to the rotor) or a value which causes a brakingforce to be applied to the rotor.

In some embodiments, the electrical connectors (704) are reversibleconnectors so that (if desired) the phase-shifted sinusoidal inputs(702) can again be passed through to the rotor inputs (706). Forexample, a switch (which can be opened or closed) can be reversed.Alternatively, the electrical connectors may comprise irreversibleconnectors, such as a fuse which would need to be replaced once it isblown.

The exemplary system is attractive for a number of reasons. First, it isrelatively inexpensive and lightweight, both of which are attractive inlow-cost, (ultra) lightweight aircraft applications. It is also simpleto implement. Again, for aircraft applications, simplicity is desirablebecause it is less likely to fail. Lastly, the system is relativelyfast. Switching the electrical connectors 704 is relatively fast, so theaircraft can respond quickly once a rotor strike is detected (which isdesirable in case a rotor is striking a person).

The following figure shows another embodiment, where existing componentsin the motor controller are used to reduce a striking force when a rotorstrike is detected.

FIG. 7B is a diagram illustrating an embodiment of built-in switches ina switched converter which are used to adjust a control signal to therotor in order to reduce a striking force. In this example, the motorcontroller (720) includes a switched converter. A switched converter isa type of circuit which is used to convert one voltage (e.g., 10 V) intoanother voltage (e.g., 5 V) using switches. The exemplary switchedcontroller actually has a total of three pairs of switches (e.g., eachof which generates a corresponding phase-shifted, sinusoidal signal) butto preserve the readability of the figure, only a single pair ofswitches is shown here.

To generate a 5 V signal from a 10 V power supply (as an example), theswitches are opened and closed in a specific pattern. First, powerswitch 724, which connects the output signal (i.e., the relevant one ofthe adaptive control signals) to the 10 V power supply, is closed forsome time t while ground switch 726 (which connects the output signal toground) is open. With the switches in this position, the output signalis connected to the 10 V power supply and not ground so that the outputsignal is at 10 V for a duration of t.

Then, the switches are reversed for the same amount of time t: the powerswitch (724) is opened so that the output is electrically disconnectedfrom the 10 V power supply, and the ground switch (726) is closed sothat the output signal is electrically connected to ground. The causesthe output signal to be 0 V for a duration of t.

This process is repeated so that a square wave with a 50% duty cycle isproduced which alternates between 10 V and 0 V which corresponds to a 5V signals. These phase-shifted, sinusoidal signals (722) produced arethen passed to the rotor, where the amplitude and phase control therotor (e.g., the torque and rotational speed, respectively).

If a strike is detected, these built-in switches can be used to reducingthe striking force of rotor 728. In some embodiments, both the powerswitch (724) and ground switch (726) are opened so that the outputsignal (i.e., one of the phase-shifted, sinusoidal signals) is connectedto neither power nor ground so that it is floating). This floating valuewould cause the rotor to gradually come to a stop (e.g., with inertiacausing the rotor to continue rotating, at least at first). Although therotor is still spinning (at least at first), the striking force isreduced and there is no sudden generation of heat.

Alternatively, if a sudden, braking stop is desired, the ground switch(726) can be closed and the power switch (724) can be opened so that theoutput signal (i.e., one of the phase-shifted, sinusoidal signals) isconnected to ground. This would cause the rotor to come to a sudden,braking stop. The downside is that a large amount of heat would begenerated (e.g., there may be a burning smell).

A benefit to using built-in switches in a switched converter is that itdoes not require new or additional parts since the motor controlleralready includes a switched converter. In contrast, even though theelectrical connectors (704) and pull-down resistors (710) in FIG. 7A maybe relatively small, cheap, and/or lightweight, they are still new oradditional parts which were not originally in the design.

The following figures describe these examples more generally and/orformally in flowcharts.

FIG. 8A is a flowchart illustrating an embodiment of a process to adjusta control signal to the rotor in order to reduce a striking force usinga pull resistor. In some embodiments, the example process shown here isused at step 102 in FIG. 1.

At 800, in response to detecting the rotor strike, an electricalconnector is used to electrically disconnect the adaptive control signalfrom a rotor input signal. See, for example, electrical connectors 704in FIG. 7A which use the rotor strike detected signal as the control orselect signal. In that example, the adaptive control signal comprisesthree phase-shifted sinusoidal signals but, naturally, in some otherembodiments, the adaptive control signal may comprise something else. Asdescribed above, an electrical connector may be a switch, a fuse, etc.

At 802, in response to electrically disconnecting the adaptive controlsignal from the rotor input signal, a pull resistor is used to set therotor input signal to a known value associated with reducing thestriking force associated with the rotor. See, for example, pull-downresistors 710 in FIG. 7A. If the electrical connectors (704)electrically disconnect the phase-shifted sinusoidal signals (702) fromthe rotor inputs (706), then the pull-down resistors will bring therotor inputs (706) to a known value, specifically zero or ground. A pullresistor (e.g., referred to in step 802) may in some other embodimentsbe a pull-up resistor connected to power.

In an aircraft application, what constitutes an appropriate response toa rotor strike being detected (e.g., at step 102 in FIG. 1) may dependupon the altitude of the aircraft when a rotor strike is detected. Thefollowing figures show an example of this, where an aircraft's altitudeis taken into consideration.

FIG. 8B is a flowchart illustrating an embodiment of processes to adjusta control signal to the rotor in order to reduce a striking force usingbuilt-in switches in a switched converter. In some embodiments, one ofthe example processes shown here is used at step 102 in FIG. 1.

At 810, in response to detecting the rotor strike, a power switch in aswitched converter is opened, wherein the power switch is connected to apower supply at one end and the adaptive control signal at the other endsuch that opening the power switch electrically disconnects the adaptivecontrol signal from the power supply. For example, motor controller 720in FIG. 7B includes a switched controller with three pairs of switches(not all of which are shown). All of the power switches (724) would beopened so that adaptive control signals (722) are not connected to thepower supply.

For brevity, two processes are shown here, where the ground switch(e.g., ground switch 726 in FIG. 7B) can either be open (e.g., so thatthe adaptive control signal is floating) or closed (e.g., so that theadaptive control signal is connected to ground) as described above.

At 812, in response to detecting the rotor strike, a ground switch inthe switched converter is closed, wherein the ground switch is connectedto ground at one end and the adaptive control signal at the other endsuch that closing the power switch electrically connects the adaptivecontrol signal to ground. For example, in FIG. 7B, all of the groundswitches (726) would be closed. This would cause all of the adaptivecontrol signals (722) to be low (i.e., at the ground value, which wouldbring rotor 728 to a sudden and/or braking halt).

Alternatively, at 814, in response to detecting the rotor strike, aground switch in the switched converter is opened, wherein the groundswitch is connected to ground at one end and the adaptive control signalat the other end such that opening the power switch electricallydisconnects the adaptive control signal from ground. For example, inFIG. 7B, all of the ground switches (726) would be opened. This wouldcause all of the adaptive control signals (722) to be floating (e.g.,which would bring rotor 728 to a gradual stop).

In some embodiments, altitude and/or other state information (e.g.,associated with the aircraft overall, as opposed to the state of one ofthe rotors) is used in deciding how to respond if a rotor strike isdetected. The following figures show some examples of this.

FIG. 9A is a diagram illustrating an embodiment of a single altitudethreshold which is used in responding to a detected rotor strike. In theexample shown, aircraft 900 is at a relatively low altitude (902) wherea hard landing is relatively safe (e.g., for the pilot and/or withrespect to damage to the aircraft). For example, the low altitude (902)may be on the order of 5-10 feet off the ground. In this example,because a hard landing is relatively safe at this altitude (902), theresponse to a rotor strike detection can favor (if desired) reducing thestriking force of the rotor, even at the expensive of the aircraft'sability to remain airborne.

In contrast, aircraft 904 is at a relatively high altitude (906), forexample, on the order of tens or hundreds of feet off the ground. Atthis altitude, a hard landing is unsafe for the pilot and the aircraft'sability to remain airborne is a more important consideration than whenthe aircraft is at the low altitude (902). At this high altitude (906),the response to a rotor strike detection takes into account both thedesire to reduce the striking force of the rotor, as well as keeping theaircraft airborne (e.g., at least to a degree necessary for a safelanding).

For example, suppose a rotor strike is detected. The aircraft's altitudeat that time is obtained, for example, using GPS or a downward-facingsensor such as radar, sonar, or lidar (e.g., which is not dependent uponhaving a good line-of-sight and/or communication channel to a GPSsatellite). The obtained altitude is compared against altitude threshold908. If the aircraft's altitude when the rotor strike is detected isbelow the altitude threshold (908), the rotor's striking force isreduced in a semi-permanent manner, for example, at least until therotor and/or some associated components or electronics are reset orotherwise replaced. For example, if electronic connectors 704 in FIG. 7comprise fuses and the fuses were blown (e.g., because the aircraft'saltitude at the time of the rotor strike was below the altitudethreshold), then the aircraft would need to land and the fuses wouldneed to be replaced before that rotor could rotate again. Alternatively,the aircraft could land and the rotor and/or its associated componentsor electronics could be reset through some user interface. In otherwords, reducing a rotor's striking force in a semi-permanent manner willprevent the rotor from restarting midair since it will need to land andcomponents will need to be replaced and/or reset for the rotor to beoperational again.

At higher altitudes, the ability to restart a rotor midair (sometimesreferred to herein as a flying restart) may be desirable in order tokeep the aircraft airborne or at least slow the descent of the aircraftto the ground. In this example, if a rotor strike is detected at analtitude above altitude threshold 908, the rotor's striking force isreduced temporarily. For example, immediately after a rotor strike isdetected at such altitudes, the rotor is slowed down or otherwisestopped (e.g., using a neutral value or a braking force). If desired,the rotor is permitted to gradually start up again, beginning at arelatively low RPM or torque and then gradually increasing the RPM ortorque. For example, a step function of gradually increasing RPMs ortorques may be specified or otherwise passed to the rotor. If, at anypoint during the step function, another rotor strike is detected, thestep function will stop and the rotor's striking force will again bereduced (e.g., again temporarily). This may permit the aircraft toreduce the striking force at least temporarily (e.g., giving the pilotand/or the person or thing being struck time to separate) while stillgetting at least some lift out of the affected rotor as opposed to nolift from the rotor once a rotor strike is detected.

In some embodiments, for safety reasons, a flying restart (which in thisexample assumes that the aircraft's altitude is above the altitudethreshold) is only permitted if it is triggered or otherwise initiatedby a pilot. For example, it may be dangerous to let some automatedprocess initiate a flying restart (e.g., in case the person or objectbeing struck is still within striking distance of one of the rotors). Incontrast, a pilot could visually inspect the area around the aircraftafter a rotor strike is detected, visually verify that the areasurrounding the aircraft is clear, and then restart the affected rotor(if desired).

In some embodiments, a user interface displays or otherwise presents aflying restart button in response to a rotor strike being detected inorder to more quickly facilitate a flying restart (e.g., so that thepilot does not have to navigate through a sequence of pages or screensin order to find the correct page or screen). In some embodiments, sucha button is presented via a touchscreen display. In some embodimentswhere the display is not a touchscreen display, the display may indicatesome sequence or combination of inputs via the hand controls which wouldbe interpreted as an instruction to perform a flying restart.

In some embodiments, multiple altitude thresholds are used. Thefollowing figure shows an example of this.

FIG. 9B is a diagram illustrating an embodiment of two altitudethresholds which are used in responding to a detected rotor strike. Inthis example, there are three altitude ranges or zones. In the lowestrange of altitudes (e.g., below the lower altitude threshold (922)), ahard landing and/or crash is not likely to be deadly or dangerous forthe pilot (e.g., if one rotor is stopped due to a strike being detectedon that rotor or otherwise goes out). Also, in this range of altitudes,the multicopter could have hit a person. For this reason, in thisexample, the striking force is reduced on a given rotor if a strike isdetected on that rotor (e.g., per the process of FIG. 1), for examplegradually or using a sudden, braking stop as desired.

In middle range of altitudes 924 (i.e., between the lower altitudethreshold (922) and higher altitude threshold (926), the multicopter isat a height or altitude where it is unlikely to strike a person. Also,in this range of altitudes, even if the aircraft has a ballisticrecovery system (i.e., a parachute system which uses a rocket or otherballistic system to help the parachute inflate), the aircraft is too lowto the ground for the ballistic recovery system to sufficiently slowdown the aircraft. In other words, a hard landing or crash in this rangeof altitudes could be dangerous or deadly to the pilot, even if theaircraft is equipped with a ballistic recovery system (or, moregenerally, a parachute or recovery system, including non-ballistic onesthat do not use rockets or ballistics to inflate). For these reasons, inthis example, in this middle range of altitudes, the aircraft “powersthrough” and does not reduce the striking force of a rotor, even if astrike is detected at that rotor.

In the highest range of altitudes (928) above the higher altitudethreshold (926), the aircraft is unlikely to strike a person and aballistic recovery system would have enough time (if deployed) to slow afalling aircraft down sufficiently to land relatively safely. For thesereasons, in this example, in this highest range of altitudes, thestriking force is reduced (e.g., per the process of FIG. 1).

In some embodiments (e.g., due to the likelihood of striking a person inthe highest range of altitudes and lowest range of altitudes), thestriking force of a rotor is reduced using a sudden, braking stop in thelowest range of altitudes (920) if a rotor strike is detected, whereas agradual stop is used in the highest range of altitudes (928). Bothreduce the striking force of the striking rotor, but rotor is brought toa stop after different durations.

In some embodiments, the direction of movement is also used to respondto a strike detection. For example, suppose that instead of ascendingstraight up through the lowest range of altitudes, the pilot decides tofly low to the ground or takes off in a diagonal manner (e.g.,simultaneously moving upwards and forwards). If so, a strike is morelikely to occur in the leading rotors. In some embodiments, the responseto a detected rotor strike depends upon whether the rotor is a leadingrotor (e.g., one of the front rotors if the aircraft is flying forwards)or a non-leading rotor (e.g., one of the side or back rotors if theaircraft if flying forwards). In some embodiments, a leading rotor isbrought to a sudden, braking halt if a strike is detected, whereas anon-leading rotor is gradually brought to a stop. This may minimize heatgeneration due to sudden, braking stops of a rotor.

These examples are described more formally and/or generally inflowcharts below.

FIG. 10A is a flowchart illustrating an embodiment of a process toadjust the control signal to the rotor in order to reduce the strikingforce associated with the rotor using altitude and an altitudethreshold. In some embodiments, the process of FIG. 10A is used at step102 in FIG. 1.

At 1000, in response to detecting the rotor strike, an altitudeassociated with the aircraft is obtained. For example, as describedabove, a variety of sensors including GPS, radar, sonar, lidar, and suchmay be used to obtain the altitude of the aircraft.

At 1002, the altitude is compared against an altitude threshold. Asdescribed above, in various embodiments, the altitude threshold may beused to differentiate between the type of object being struck (e.g., atlow altitudes there is a high(er) likelihood of striking a personwhereas at high altitudes there is a low(er) likelihood of striking aperson) or a safe versus unsafe height at which to have a hard or crashlanding. In some embodiments, two or more altitude thresholds are used.

At 1004, in response to the altitude exceeding the altitude threshold,the control signal to the rotor is at least temporarily adjusted inorder to reduce the striking force associated with the rotor. Forexample, the affected rotor may be permitted (e.g., under certainconditions, after a certain amount of time, if so instructed by a pilot,etc.) to do a flying restart (e.g., without landing and/or withoutresetting or replacing something). As described above, it may be saferto let the rotor restart at high altitudes in order to prevent orotherwise mitigate a hard or crash landing.

At 1006, in response to the altitude not exceeding the altitudethreshold, the control signal to the rotor is adjusted in order toreduce the striking force associated with the rotor, at least until oneor more of the following occurs: a reset or a part replacement. Forexample, the affected rotor would not be permitted to restart untilthere was a reset or a part was replaced. Presumably this happens on theground and presumably this happens after the person or object beingstruck is no longer in danger of being struck.

FIG. 10B is a flowchart illustrating an embodiment of a process toadjust the control signal to the rotor in order to reduce the strikingforce associated with the rotor using altitude and an altitude thresholdwhere a pilot is permitted to restart the rotor. In some embodiments,the process of FIG. 10B is used at step 102 in FIG. 1. FIG. 10B issimilar to FIG. 10A and the same or similar reference numbers are usedto indicate the same or similar steps.

At 1000, in response to detecting the rotor strike, an altitudeassociated with the aircraft is obtained.

At 1002, the altitude is compared against an altitude threshold.

At 1004′, in response to the altitude exceeding the altitude threshold,the control signal to the rotor is at least temporarily adjusted inorder to reduce the striking force associated with the rotor, whereinthe rotor is permitted to restart in response to a pilot restartinstruction without a reset prior to the pilot restart instruction or apart replacement prior to the pilot restart instruction. As describedabove, it may be safer to let a pilot control any flying restart becausethe pilot can make a visual inspection and confirm there is nothingwhich would be hit by the affected rotor if that rotor were restarted.

At 1006, in response to the altitude not exceeding the altitudethreshold, the control signal to the rotor is adjusted in order toreduce the striking force associated with the rotor, at least until oneor more of the following occurs: a reset or a part replacement.

FIG. 10C is a flowchart illustrating an embodiment of a process toadjust the control signal to the rotor in order to reduce the strikingforce associated with the rotor using two altitude thresholds when thereis a parachute system. In some embodiments, the process of FIG. 10C isused at step 102 in FIG. 1.

At 1010, in response to detecting the rotor strike, an altitudeassociated with the aircraft is obtained.

At 1012, the altitude is compared against a higher altitude thresholdand a lower altitude threshold. As described above, in some embodiments,below the lower altitude threshold, the aircraft may be able to crashsafely and includes altitudes where the aircraft may strike people. Insome embodiments, the higher altitude threshold may be associated with acutoff altitude above which a (e.g., ballistic) recovery system is ableto sufficiently slow an aircraft down for a safe landing (and belowwhich, the recovery system would not have enough time to deploy and slowthe aircraft down sufficiently).

At 1014, in response to the altitude not exceeding the lower altitudethreshold, the control signal to the rotor is adjusted in order toreduce the striking force associated with the rotor. See, for example,the description of how the system responds in the lowest range ofaltitudes (920) in FIG. 9B.

At 1016, in response to the altitude exceeding the higher altitudethreshold, the control signal to the rotor is adjusted in order toreduce the striking force associated with the rotor, wherein in responseto the altitude exceeding the lower altitude threshold and not exceedingthe higher altitude threshold, the striking force associated with therotor is not reduced. See, for example, the description for how theexemplary aircraft in FIG. 9B responds to a rotor strike being detectedin the highest range of altitudes (928) and the middle range ofaltitudes (924).

In some embodiments, as described above, there is furtherdifferentiation in the lowest altitude threshold (e.g., for leadingrotors versus non-leading rotors). In some embodiments, rotors in thelowest altitude threshold are stopped immediately if a rotor strike isdetected, whereas rotors in the highest altitude threshold are stoppedgradually if a rotor strike is detected (e.g., where both approacheswould still reduce the striking force of the rotor).

Although the foregoing embodiments have been described in some detailfor purposes of clarity of understanding, the invention is not limitedto the details provided. There are many alternative ways of implementingthe invention. The disclosed embodiments are illustrative and notrestrictive.

What is claimed is:
 1. A system, comprising: a processor; and a memory coupled with the processor, wherein the memory is configured to provide the processor with instructions which when executed cause the processor to: detect a rotor strike of an object striking an aircraft rotor, the detection including comparing a commanded control signal against an adaptive control signal including by: determining a degree to which the adaptive control signal deviates from a reference signal; comparing the degree of deviation against a detection threshold; and in response to the degree exceeding the detection threshold, declaring that the rotor strike has been detected, wherein the adaptive control signal is associated with controlling the rotor and the adaptive control signal varies based at least in part on the commanded control signal and state information associated with the rotor; and in response to detecting the rotor strike, adjust a control signal to the rotor to reduce a striking force applied to the object being struck by the rotor and mitigate damage to at least one of the object being struck and the rotor.
 2. The system recited in claim 1, wherein the instructions for comparing the commanded is control signal against the adaptive control signal in order to detect the rotor strike include instructions for determining a reference signal based at least in part on the commanded control signal and the adaptive control signal includes a sinusoidal signal.
 3. The system recited in claim 1, wherein the instructions for comparing the commanded control signal against the adaptive control signal in order to detect the rotor strike include instructions for determining a reference signal based at least in part on the commanded control signal, wherein the reference signal includes a measurement threshold.
 4. The system recited in claim 1, wherein the instructions for comparing the commanded control signal against the adaptive control signal in order to detect the rotor strike include instructions for determining a reference signal based at least in part on the commanded control signal, wherein the reference signal includes a sinusoidal reference signal.
 5. The system recited in claim 1, wherein the instructions for adjusting the control signal to the rotor in order to reduce the striking force include instructions for: in response to detecting the rotor strike, using an electrical connector to electrically disconnect the adaptive control signal from a rotor input signal; and in response to electrically disconnecting the adaptive control signal from the rotor input signal, using a pull resistor to set the rotor input signal to a known value associated with reducing the striking force associated with the rotor.
 6. The system recited in claim 1, wherein the instructions for adjusting the control signal to the rotor in order to reduce the striking force include instructions for: in response to detecting the rotor strike, opening a power switch in a switched converter, wherein the power switch is connected to a power supply at one end and the adaptive control signal at the other end such that opening the power switch electrically disconnects the adaptive control signal from the power supply; and in response to detecting the rotor strike, closing a ground switch in the switched converter, wherein the ground switch is connected to ground at one end and the adaptive control signal at the other end such that closing the power switch electrically connects the adaptive control signal to ground.
 7. The system recited in claim 1, wherein the instructions for adjusting the control signal to the rotor in order to reduce the striking force include instructions for: in response to detecting the rotor strike, opening a power switch in a switched converter, wherein the power switch is connected to a power supply at one end and the adaptive control signal at the other end such that opening the power switch electrically disconnects the adaptive control signal from the power supply; and in response to detecting the rotor strike, opening a ground switch in the switched converter, wherein the ground switch is connected to ground at one end and the adaptive control signal at the other end such that opening the power switch electrically disconnects the adaptive control signal from ground.
 8. The system recited in claim 1, wherein the instructions for adjusting the control signal to the rotor in order to reduce the striking force include instructions for: in response to detecting the rotor strike, obtaining an altitude associated with the aircraft; comparing the altitude against an altitude threshold; in response to the altitude exceeding the altitude threshold, at least temporarily adjusting the control signal to the rotor in order to reduce the striking force associated with the rotor; and in response to the altitude not exceeding the altitude threshold, adjusting the control signal to the rotor in order to reduce the striking force associated with the rotor, at least until one or more of the following occurs: a reset or a part replacement.
 9. The system recited in claim 1, wherein the instructions for adjusting the control signal to the rotor in order to reduce the striking force include instructions for: in response to detecting the rotor strike, obtaining an altitude associated with the aircraft; comparing the altitude against an altitude threshold; in response to the altitude exceeding the altitude threshold, at least temporarily adjusting the control signal to the rotor in order to reduce the striking force associated with the rotor, wherein the rotor is permitted to restart in response to a pilot restart instruction without a reset prior to the pilot restart instruction or a part replacement prior to the pilot restart instruction; and in response to the altitude not exceeding the altitude threshold, adjusting the control signal to the rotor in order to reduce the striking force associated with the rotor, at least until one or more of the following occurs: a reset or a part replacement.
 10. The system recited in claim 1, wherein: the aircraft includes a parachute system; and the instructions for adjusting the control signal to the rotor in order to reduce the striking force include instructions for: in response to detecting the rotor strike, obtaining an altitude associated with the aircraft; comparing the altitude against a higher altitude threshold and a lower altitude threshold; in response to the altitude not exceeding the lower altitude threshold, adjusting the control signal to the rotor in order to reduce the striking force associated with the rotor; and in response to the altitude exceeding the higher altitude threshold, adjusting the control signal to the rotor in order to reduce the striking force associated with the rotor, wherein in response to the altitude exceeding the lower altitude threshold and not exceeding the higher altitude threshold, the striking force associated with the rotor is not reduced.
 11. A method, comprising: detecting a rotor strike of an object striking an aircraft rotor, the detection including comparing a commanded control signal against an adaptive control signal including by: determining a degree to which the adaptive control signal deviates from a reference signal; comparing the degree of deviation against a detection threshold; and in response to the degree exceeding the detection threshold, declaring that the rotor strike has been detected, wherein the adaptive control signal is associated with controlling the rotor and the adaptive control signal varies based at least in part on the commanded control signal and state information associated with the rotor; and in response to detecting the rotor strike, adjusting a control signal to the rotor to reduce a striking force applied to the object being struck by the rotor and mitigate damage to at least one of the object being struck and the rotor.
 12. The method recited in claim 11, wherein comparing the commanded control signal against the adaptive control signal in order to detect the rotor strike includes determining a reference signal based at least in part on the commanded control signal and the adaptive control signal includes a sinusoidal signal.
 13. The method recited in claim 11, wherein comparing the commanded control signal against the adaptive control signal in order to detect the rotor strike includes determining a reference signal based at least in part on the commanded control signal, wherein the reference signal includes a measurement threshold.
 14. The method recited in claim 11, wherein comparing the commanded control signal against the adaptive control signal in order to detect the rotor strike includes determining a reference signal based at least in part on the commanded control signal, wherein the reference signal includes a sinusoidal reference signal.
 15. The method recited in claim 11, wherein adjusting the control signal to the rotor in order to reduce the striking force includes: in response to detecting the rotor strike, using an electrical connector to electrically disconnect the adaptive control signal from a rotor input signal; and in response to electrically disconnecting the adaptive control signal from the rotor input signal, using a pull resistor to set the rotor input signal to a known value associated with reducing the striking force associated with the rotor.
 16. The method recited in claim 11, wherein adjusting the control signal to the rotor in order to reduce the striking force includes: in response to detecting the rotor strike, opening a power switch in a switched converter, wherein the power switch is connected to a power supply at one end and the adaptive control signal at the other end such that opening the power switch electrically disconnects the adaptive control signal from the power supply; and in response to detecting the rotor strike, closing a ground switch in the switched converter, wherein the ground switch is connected to ground at one end and the adaptive control signal at the other end such that closing the power switch electrically connects the adaptive control signal to ground.
 17. The method recited in claim 11, wherein adjusting the control signal to the rotor in order to reduce the striking force includes: in response to detecting the rotor strike, opening a power switch in a switched converter, wherein the power switch is connected to a power supply at one end and the adaptive control signal at the other end such that opening the power switch electrically disconnects the adaptive control signal from the power supply; and in response to detecting the rotor strike, opening a ground switch in the switched converter, wherein the ground switch is connected to ground at one end and the adaptive control signal at the other end such that opening the power switch electrically disconnects the adaptive control signal from ground.
 18. The method recited in claim 11, wherein adjusting the control signal to the rotor in order to reduce the striking force includes: in response to detecting the rotor strike, obtaining an altitude associated with the aircraft; comparing the altitude against an altitude threshold; in response to the altitude exceeding the altitude threshold, at least temporarily adjusting the control signal to the rotor in order to reduce the striking force associated with the rotor; and in response to the altitude not exceeding the altitude threshold, adjusting the control signal to the rotor in order to reduce the striking force associated with the rotor, at least until one or more of the following occurs: a reset or a part replacement.
 19. The method recited in claim 11, wherein adjusting the control signal to the rotor in order to reduce the striking force includes: in response to detecting the rotor strike, obtaining an altitude associated with the aircraft; comparing the altitude against an altitude threshold; in response to the altitude exceeding the altitude threshold, at least temporarily adjusting the control signal to the rotor in order to reduce the striking force associated with the rotor, wherein the rotor is permitted to restart in response to a pilot restart instruction without a reset prior to the pilot restart instruction or a part replacement prior to the pilot restart instruction; and in response to the altitude not exceeding the altitude threshold, adjusting the control signal to the rotor in order to reduce the striking force associated with the rotor, at least until one or more of the following occurs: a reset or a part replacement.
 20. The method recited in claim 11, wherein: the aircraft includes a parachute system; and adjusting the control signal to the rotor in order to reduce the striking force includes: in response to detecting the rotor strike, obtaining an altitude associated with the aircraft; comparing the altitude against a higher altitude threshold and a lower altitude threshold; in response to the altitude not exceeding the lower altitude threshold, adjusting the control signal to the rotor in order to reduce the striking force associated with the rotor; and in response to the altitude exceeding the higher altitude threshold, adjusting the control signal to the rotor in order to reduce the striking force associated with the rotor, wherein in response to the altitude exceeding the lower altitude threshold and not exceeding the higher altitude threshold, the striking force associated with the rotor is not reduced.
 21. A computer program product, the computer program product being embodied in a non-transitory computer readable storage medium and comprising computer instructions for: detect a rotor strike of an object striking an aircraft rotor, the detection including comparing a commanded control signal against an adaptive control signal including by: determining a degree to which the adaptive control signal deviates from a reference signal; comparing the degree of deviation against a detection threshold; and in response to the degree exceeding the detection threshold, declaring that the rotor strike has been detected, wherein the adaptive control signal is associated with controlling the rotor and the adaptive control signal varies based at least in part on the commanded control signal and state information associated with the rotor; and in response to detecting the rotor strike, adjust a control signal to the rotor to reduce a striking force applied to the object being struck by the rotor and mitigate damage to at least one of the object being struck and the rotor. 